1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor for a magnetic read head, more specifically to the use of canted synthetic exchange biasing to produce a sensor with increased dynamic range, increased stability and improved control of its bias point.
2. Description of the Related Art
Magnetic read heads whose sensors make use of the giant magnetoresistive effect (GMR) in the spin-valve configuration (SVMR) base their operation on the fact that magnetic fields produced by data stored in the medium being read cause the direction of the magnetization of one layer in the sensor (the free magnetic layer) to move relative to a fixed magnetization direction of another layer of the sensor (the fixed or pinned magnetic layer). Because the resistance of the sensor element is proportional to the cosine of the (varying) angle between these two magnetizations, a constant current (the sensing current) passing through the sensor produces a varying voltage across the sensor which is interpreted by associated electronic circuitry. The accuracy, linearity and stability required of a GMR sensor places stringent requirements on the magnetization of its fixed and free magnetic layers. The fixed layer, for example, has its magnetization “pinned” in a direction normal to the air bearing surface of the sensor (the transverse direction) by an adjacent magnetic layer called the pinning layer. The free layer is magnetized in a direction along the width of the sensor and parallel to the air bearing surface (the longitudinal direction). The prior art also teaches dual sensors, such as is taught by Gill et al. (U.S. Pat. No. 5,701,222) wherein two identical sensor structures are formed, one on top of the other, differing only in that the magnetizations of their fixed layers are antiparallel.
Layers of hard magnetic material (permanent magnetic layers) or laminates of antiferromagnetic and soft magnetic materials are typically formed on each side of a sensor and oriented so that their magnetic field extends in the same direction as that of the free layer. These layers, called longitudinal bias layers, maintain the free layer as a single magnetic domain and also assist in linearizing the sensor response by keeping the free layer magnetization direction normal to that of the fixed layer when the sensor is quiescent (not reading data). Maintaining the free layer in a single domain state significantly reduces noise (Barkhausen noise) in the signal produced by thermodynamic variations in domain configurations.
The importance of effective longitudinal bias has led to various inventions designed to improve the material composition, structure, positioning and method of forming the magnetic layers that produce it. One form of the prior art provides for sensor structures in which the longitudinal bias layers are layers of hard magnetic material (permanent magnets) that abut the etched back ends of the active region of the sensor to produce what is called an abutted junction configuration. This arrangement fixes the domain structure of the free magnetic layer by magnetostatic coupling through direct edge-to-edge contact at the etched junction between the biasing layer and the exposed end of the layer being biased (the free layer). Another form of the prior art, patterned exchange bias, appears in two versions: 1) direct exchange and 2) synthetic exchange. Unlike the magnetostatic coupling resulting from direct contact with a hard magnetic material that is used in the abutted junction, in exchange coupling the free layer is extended laterally beyond the trackwidth region. This outer extended region is called the “wing region.” The magnetization in the wing region is fixed by a biasing layer which overlays the wing region of the free layer. This biasing layer is either a single layer of antiferromagnetic material, in the direct exchange scheme, or a synthetic antiferromagnetic layer in the synthetic exchange scheme. In direct exchange coupling, an antiferromagnetic material such as IrMn, PtMn, or NiMn is directly overlaid on the free layer in the wing region in a simple scheme, but one with weak pinning strength. In synthetic exchange coupling, a synthetic antiferromagnetic biasing layer is formed by separating two ferromagnetic layers by a non-magnetic coupling layer (eg. Cu, Ru or Rh) whose thickness is chosen to allow antiferromagnetic coupling, wherein the magnetization of the biasing and biased layers are antiparallel. Xiao et al. (U.S. Pat. No. 6,322,640 B1) disclose a method for forming a double, antiferromagnetically biased GMR sensor, using as the biasing material a magnetic material having two crystalline phases, one of which couples antiferromagnetically and the other of which does not. Liao et al. (U.S. Pat. No. 6,308,400 B1) teach a method of achieving anti-parallel exchange coupling by the use of a biased layer with low coercivity. The use of novel forms of direct and synthetic exchange coupling in providing longitudinal biasing of a sensor is taught in related Patent Applications HT-01-037, and HT-01-032 assigned to the same assignee as the present invention and which is fully incorporated herein by reference. HT-01-032 teaches direct exchange coupling using an antiferromagnetic layer as the biasing layer. Related application HT-01-037, also assigned to the same assignee as the present invention, teaches synthetic exchange coupling using antiferromagnetic exchange coupling between the biasing layer and the free layer. The use of synthetic exchange coupling in providing both longitudinal and transverse biasing (“transverse” meaning pinning the free layer transversely at its lateral edges, but maintaining its longitudinal magnetization in the sensor trackwidth region) of a sensor is taught in related Patent Application HT-01-036/038 assigned to the same assignee as the present invention and which is fully incorporated herein by reference.
The discussion above has centered on various methods of providing longitudinal and transverse biasing of a free layer. Along with the choice of method, the practitioner skilled in the art has the additional freedom of biasing the free magnetic layer so that its magnetization is in a direction other than perpendicular to or transverse to the plane of the air bearing surface of the sensor. Indeed, the prior art teaches canted biasing in the context of direct exchange biasing, wherein magnetic layers are biased at various angles to the air bearing surface in order to improve sensor performance. Li et al. (U.S. Pat. No. 6,295,718 B1) teaches a method of fabricating a sensor having multiple magnetic layers that are exchange biased in non-parallel directions, while still using a single biasing material, but employing a series of magnetic annealing steps. The method discloses an enhanced bias profile that is provided by the non-parallel biasing directions. In a somewhat similar vein, Guo et al. (U.S. Pat. No. 6,230,390 B1) teaches a method of forming a dual stripe sensor (one sensor element formed over another) in which the free layers of each sensor are directly exchange biased in directions canted relative to the air bearing surface and relative to each other.
As the area density of magnetization in magnetic recording media (eg. magnetic disks) continues to increase (eg. above 30 gigabytes/in2), significant reduction in the width of the active sensing region (trackwidth) of read-sensors becomes necessary. For trackwidths less than 0.2 microns (μm), the traditional abutted junction hard bias structure discussed above becomes unsuitable because the strong magnetostatic coupling at the junction surface actually pins the magnetization of the (very narrow) biased layer (the free layer), making it less responsive to the signal being read and, thereby, significantly reducing the sensor sensitivity. Under such very narrow trackwidth conditions, the exchange bias method becomes increasingly attractive, since the free layer is not reduced in size by the formation of an abutted junction, but extends continuously across the entire width of the sensor element.
The direct exchange biasing also has its shortcomings when used in a very narrow trackwidth configuration because of the weakness of the pinning field. For example, the pinning field provided to the free and biasing layers by the antiferromagnetic layer in HT-01-032 cited above is found to be, typically, approximately 250 Oe. A stronger pinning field, typically exceeding 700 Oe, can be obtained using the synthetic exchange biasing method. As noted above, related Patent Applications HT-01-037 and HT-01-036/38 both teach methods of forming synthetic exchange (longitudinally or transversely) biased sensors in which the sensor's free layer is strongly pinned by the exchange biasing layers, yet in which a narrow trackwidth can be formed. It is the purpose of the present invention to teach a method of canting the biasing magnetizations within the context of the synthetic exchange biasing taught in the related Patent Applications above and to thereby further improve the performance of the sensor by eliminating instability and improving the bias point.